U.S. patent application number 09/345813 was filed with the patent office on 2002-04-04 for composites of powdered fillers and polymer matrix.
Invention is credited to HOLL, RICHARD A..
Application Number | 20020038582 09/345813 |
Document ID | / |
Family ID | 23356603 |
Filed Date | 2002-04-04 |
United States Patent
Application |
20020038582 |
Kind Code |
A1 |
HOLL, RICHARD A. |
April 4, 2002 |
COMPOSITES OF POWDERED FILLERS AND POLYMER MATRIX
Abstract
Composite materials comprising at least 60 volume %, preferably
70 volume %, of particles of finely powdered filler material in a
matrix of poly(arylene ether) polymer material are made by forming
a mixture of the components, forming the required bodies therefrom,
and then heating and pressing the bodies to a temperature
sufficient to melt the polymer and to a pressure sufficient to
disperse the melted polymer into the interstices between the filler
particles. Surprisingly these polymer materials can only be
effective as bonding materials when the solids content is as high
as that specified, since with lower contents the resultant bodies
are too friable. This is completely contrary to accepted prior art
practice which considers that composites are progressivly weakened
as the solids content is increased, so that such content must be
limited. In processes to obtain as complete a dispersion of the
components as possible they are individually dispersed in a liquid
dispersion medium containing the polymer together with necessary
additives, each mixture being ground if required to obtain a
desired particle size, the mixtures are mixed, again ground to
produce thorough dispersion, are separated from the liquid
dispersion medium and green articles formed from the resulting
pasty mixture. The green articles are then heated and pressed as
described above. Mixtures of different filler materials may be used
to tailor the electrical and physical properties of the final
materials. The articles preferably comprise substrates for use in
electronic circuits.
Inventors: |
HOLL, RICHARD A.; (OXNARD,
CA) |
Correspondence
Address: |
RICHARD A HOLL
HOLL TECHNOLOGIES COMPANY
2183 EASTRIDGE TRAIL
OXNARD
CA
93030
|
Family ID: |
23356603 |
Appl. No.: |
09/345813 |
Filed: |
July 2, 1999 |
Current U.S.
Class: |
75/230 ;
257/E23.007 |
Current CPC
Class: |
B01J 2219/00094
20130101; C08J 3/212 20130101; H05K 3/381 20130101; B22F 3/20
20130101; B22F 3/227 20130101; B29K 2503/04 20130101; B29K 2021/00
20130101; H01L 23/145 20130101; H05K 2201/083 20130101; B22F
2998/00 20130101; H05K 1/0373 20130101; H05K 2203/068 20130101;
B29B 7/82 20130101; B29K 2071/00 20130101; B22F 1/16 20220101; B29B
7/603 20130101; B29C 48/07 20190201; H01L 2924/0002 20130101; C08K
3/01 20180101; B29C 48/38 20190201; B29C 67/242 20130101; B22F
1/107 20220101; H05K 2201/0792 20130101; B29B 7/90 20130101; B29B
7/7466 20130101; H01L 2924/12044 20130101; H05K 2201/0254 20130101;
B29B 7/845 20130101; B29K 2105/16 20130101; H05K 3/022 20130101;
H05K 2201/0215 20130101; B29C 70/58 20130101; H05K 3/14 20130101;
H01L 23/3737 20130101; H05K 2201/0209 20130101; H05K 2201/0212
20130101; B29C 48/00 20190201; B29B 7/7461 20130101; H01L 23/293
20130101; B22F 7/04 20130101; H01L 21/4882 20130101; B01F 33/83613
20220101; C08J 2371/12 20130101; H05K 2203/0278 20130101; B29B
13/10 20130101; H05K 2201/0227 20130101; C22C 32/0094 20130101;
H05K 2201/068 20130101; H05K 2203/1194 20130101; B29B 7/401
20130101; Y10T 428/12028 20150115; C08L 71/12 20130101; H01L
2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
75/230 |
International
Class: |
C22C 029/00 |
Claims
I claim:
1. A method of manufacturing composite materials comprising
particles of finely powdered filler material uniformly distributed
in a matrix of polymer material, the method comprising the steps
of: mixing together from 60 to 97 volume percent of particles of
the filler material and the balance non-polar polymer bonding
material consisting of nonfunctionalized poly(arylene ether) to
form a composite mixture; and subjecting the composite mixture to a
temperature sufficient to melt the polymer material and to a
pressure sufficient to uniformly disperse the melted polymer
material into the interstices between the particles of filler
material.
2. A method as claimed in claim 1, wherein the polymer material is
selected from the group comprising polyarylene ether-2, polyarylene
ether-3, and polyarylene ether-4.
3. A method as claimed in claim 2, wherein the polymer is heated to
a temperature in the range 350-450.degree. C. to obtain
cross-linking and consequent increase in molecular weight.
4. A method as claimed in claim 2, wherein the composite mixture
includes a cross-linking agent and/or an end capping agent to
facilitate a consequent increase in molecular weight of the polymer
upon heating.
5. A method as claimed in claim 1, wherein the filler material is
selected from the group comprising particles of inorganic material,
particles of electromagnetic material, particles of a core of
inorganic material covered with a layer of a metal oxide material,
particles of metal material, particles of magnetic material, and
particles of low dielectric constant high melting point polymer
material, all of which particles may be hollow.
6. A method as claimed in claim 1, wherein the composite mixture is
heated to a temperature in the range 280-400.degree. C. and to a
pressure in the range 3.5 to 1,380 MPa (500 to 200,000 psi),
preferably 70 to 1,380 MPa (10,000 to 200,000 psi).
7. A method as claimed in claim 1, comprising also the steps of:
mixing together the particles of filler material in finely powdered
form, the polymer, and a liquid dispersion medium to form a
flowable composite mixture thereof; grinding the flowable composite
mixture to uniformly disperse the particles of the finely powdered
materials in the liquid dispersion medium; removing liquid
dispersion medium from the flowable composite mixture to produce a
pasty composite mixture and forming green articles from the
composite pasty mixture; and subjecting the green articles to the
specified temperature and pressure.
8. A method as claimed in claim 7, wherein each of the polymer and
the filler material are mixed separately with the liquid respective
dispersion medium, and are mixed in respective drum type grinding
apparatus as disclosed in U.S. Pat. Nos. 5,279,463 and 5,538,191 to
provide uniform dispersion of the components.
9. A method as claimed in claim 7, wherein the composite mixture is
mixed in at least one drum type grinding apparatus as disclosed in
U.S. Pat. Nos. 5,279,463 and 5,538,191 to provide uniform
dispersion of the components.
10. A method as claimed in claim 1, wherein the particles of filler
material are of size in the range 0.1 to 50 micrometers and the
polymer material, when in the form of solid particles is also of
size in the range 0.01 to 50 micrometers, and wherein the particles
of filler material may consist of a mixture of filler materials of
different chemical compositions.
11. A method as claimed in claim 1, and including the step of
forming the heated and pressurized composite mixture into a sheet,
film or tape.
12. A method as claimed in claim 11, and including the step of
applying a layer of copper to a surface of the sheet, film or tape
by a process selected from sputtering and direct bonding of copper
foil under heat and pressure in a vacuum.
13. A method as claimed in claim 11, wherein the sheet, film or
tape has a thickness less than about 60 mil, alternatively less
than about 30 mil, alternatively less than about 10 mil,
alternatively less than about 4 mil, and alternatively less than
about 1 mil.
14. A method as claimed in claim 1, and including the step of
applying a layer of copper to a surface of the heated and
pressurized composite mixture by a process selected from sputtering
and direct bonding of copper foil.
15. A method as claimed in claim 1, and including the step of
forming substrates for electronic circuits from the heated and
pressurized composite mixture.
16. A method as claimed in claim 1, and including the step of
enclosing electronic circuits or devices with the heated and
pressurized composite mixture.
17. Composite materials comprising particles of finely powdered
filler material uniformly distributed in a matrix of polymer
material, the materials comprising: from 15 to 97 volume percent of
particles of the filler material and the balance non-polar polymer
material consisting of nonfunctionalized poly(arylene ether)
together forming a composite mixture; wherein the composite mixture
has been subjected to a temperature sufficient to melt the polymer
material and to a pressure sufficient to uniformly disperse the
melted polymer material into the interstices between the particles
of filler material.
18. Materials as claimed in claim 17, wherein the polymer material
is selected from the group comprising polyarylene ether-2,
polyarylene ether-3, and polyarylene ether-4.
19. Materials as claimed in claim 17, wherein the filler material
is selected from the group comprising particles of inorganic
material, particles of electromagnetic material, particles of a
core of inorganic material covered with a layer of a metal oxide
material, particles of metal material, particles of magnetic
material, and particles of low dielectric constant high melting
point polymer material, all of which particles may be hollow.
20. Materials as claimed in claim 17, wherein the particles of
filler material are of size in the range 0.1 to 50 micrometers, and
wherein the particles of filler material may consist of a mixture
of filler materials of different chemical compositions.
21. Materials as claimed in claim 17, and having the form of a
sheet, film or tape, and wherein the sheet, film or tape has a
thickness less than about 60 mil, alternatively less than about 30
mil, alternatively less than about 10 mil, alternatively less than
about 4 mil, and alternatively less than about 1 mil.
22. Materials as claimed in claim 21, and comprising a layer of
copper applied to a surface of the sheet, film or tape by
sputtering or by direct bonding of copper foil under heat and
pressure in a vacuum.
23. Materials as claimed in claim 17, and having a layer of copper
applied to a surface by sputtering or by direct bonding of copper
foil.
24. Materials as claimed in claim 17, and comprising substrates for
electronic circuits formed from the heated and pressurized
composite mixture.
25. Materials as claimed in claim 17, and comprising electronic
circuits or devices enclosed with the composite mixture.
Description
TECHNICAL FIELD
[0001] The invention is concerned with methods for the manufacture
of composite materials consisting of particles of finely powdered
filler material bonded in a matrix of polymer material, and new
composite materials made by such methods.
BACKGROUND ART
[0002] The electronics industry is an example of one which makes
substantial use of printed wiring boards and substrates as supports
and dielectric participants for electronic circuits, such
substrates consisting of thin flat pieces produced to exacting
specifications as to starting material and physical and electrical
properties. The history of the industry shows the use of
progressively higher operating frequencies and currently for
frequencies up to about 800 megahertz (MHz) copper coated circuit
boards of glass fiber reinforced polymers, such as epoxies, cyanide
esters, polytetrafluoroethylene (PTFE) and polyimides, have been
and are still used. At present one popular laminate material for
such applications is FR-4, consisting of epoxy resin deposited on a
woven glass fabric, because of its ease of manufacture and low
cost. Typically this material has a dielectric constant of 4.3-4.6
and a dissipation factor of 0.016-0.022 and is frequently used in
computer related applications below about 500 MHz frequencies.
Mobile telephones now operate at frequencies of 1-40 GHz and some
computers already at 0.5 GHz, with the prospect of higher
frequencies in the future. The lowest possible value of dielectric
constant is preferred in computer applications to improve signal
speed. At higher operating frequencies above approximately 0.8 GHz,
FR-4 and similar materials are materials, despite their low cost,
are no longer entirely suitable, primarily because of unacceptable
dielectric losses, heating up, lack of sufficient uniformity,
unacceptable anisotropy, unacceptable mismatch of thermal expansion
between the dielectric material and its metallization, and
anisotropic thermal expansion problems as the operating
temperatures of the substrates fluctuate. These thermal expansion
problems result from the relatively large coefficients of thermal
expansion of the polymers used as substrate material, and the
unequal expansion coefficients of reinforcing fibers in their
length and thickness dimensions. For frequencies above 800 MHz the
dielectric material of the substrates become an active capacitative
participant in signal propagation and here the current materials of
choice are certain ceramics formed by sintering or firing suitable
powdered inorganic materials, such as fused silica; alumina;
aluminum nitride; boron nitride; barium titanate; barium titanate
complexes such as Ba(Mg.sub.1/3Ti.sub.2/3)O.sub.2,
Ba(Zr,Sn)TiO.sub.4, and BaTiO.sub.3 doped with Sc.sub.2O.sub.3 and
rare earth oxides; alkoxide-derived SrZrO.sub.3 and SrTiO.sub.3;
and pyrochlore structured Ba(Zr,Nb) oxides. Substrates have also
been employed consisting of metal powders, and semiconductor
powders embedded in a glass or polymer matrix, a particular
preferred family of polymers being those based on PTFE.
[0003] For example, ceramic substrates that have been used for
hybrid electronic circuit applications comprise square plates of 5
cm (2 ins) side, their production usually involving the preparation
of a "slip" (slurry) of the finely powdered materials dispersed in
a liquid vehicle, dewatering the slip to a stiff leathery mixture,
making a "green" preform from the mixture, and then sintering the
preform to become the final substrate plate. The substrates are
required to have highly uniform values of thickness, grain size,
grain structure, density, surface flatness and surface finish, with
the purpose of obtaining uniform dielectric, thermal and chemical
properties, and also to permit the uniform application to the
surfaces of fine lines of conductive or resistive metals or
inks.
[0004] Such sintered products inherently contain a number of
special and very characteristic types of flaws. A first consists of
fine holes created by the entrainment of bubbles in the ceramic
pre-casting slip of sizes in the range about 1-20 micrometers;
these bubbles cannot be removed from the slip by known methods and
cause residual porosity in the body. As an example, sintered
alumina substrates may have as many as 800 residual bubble holes
per sq/cm of surface (5,000 per sq/in). Another flaw is
triple-point holes at the junctions of the ceramic granules when
the substrate has been formed by roll-compacting of spray-dried
powder; they are of similar size to the bubble holes and appear in
similar numbers per sq/cm. Yet another consists of "knit-lines",
which are webs or networks of seam lines of lower density formed at
the contact areas between butting particles during cold pressing.
Two other common flaws are caused by inclusions of foreign matter
into the material during processing, and the enlarged grains caused
by agglomeration of the particles despite their initial fine
grinding. The usual inclusions are fine particles due to abrasive
wear of the grinding media in the mills. Both inclusions and
agglomerates will sinter in a matrix at a different rate from the
remainder of the matrix and can result in flaws of much greater
magnitude than the original inclusion or agglomerate.
[0005] Costly mirror-finishing by diamond machining and lapping of
the ceramic surfaces is required to allow the accurate deposition
of sputtered metallization layers from which conductor lines are
formed by etching. Mirror-finishes are required because the
electrical currents at frequencies above 0.8 GHz move predominantly
in the skin of a conductor line while in the lower frequencies they
occupy the entire crossection of the conductor line. The thickness
of the skin through which currents move at GHz frquencies becomes
thinner as frequencies rise and are already as thin as 1 to 2
micrometers in copper at around 2 GHz. Any surface roughness of the
conductors on the top and bottom sides will therefor contribute to
considerable conductive losses. For example, at a frequency of 4
GHz, the conductive loss at of the interface between conductor and
substrate is 1.65 times higher at a RMS roughness of 40 compared to
a RMS roughness of 5 (See P.42 of Materials and Processes for
Microwave Hybrids, Richard Brown, published 1989 by the
International Society for Hybrid Microelectronics of Reston,
Va.)
[0006] There is therefore continuing interest in methods for
manufacturing composite materials for the production of electronic
substrates and for use as electronic packaging materials, with
which sintering and the high processing temperatures required
together with their attendant difficulties, high cost of diamond
machining and lapping, and the associated considerable costs are
avoided.
[0007] The low inherent mechanical strength of the currently
available matrix forming polymers and their excessive thermal
expansion coefficient has made it necessary to embed reinforcing
materials, such as woven glass fiber cloth, into the substrate
body, to strengthen it and also to contrain its excessive thermal
expansion. But such reinforcing materials unfortunately cause
unacceptable inhomogenity of the structure. For example, the
presence of such reinforcing material makes it difficult to
incorporate powdered filler materials uniformly into the body of
the substrate. Another difficulty is caused by the generally poor
adhesion exhibited by the commercially available matrix polymers
toward the usual filler materials, and extensive research and
development has been undertaken in the past, and is continuing, in
connection with known substrate-forming polymers, such as PTFE, to
find coupling agents that will provide adequate adhesion between
the polymer and the powder components, and thus satisfactory
mechanical strength in the resultant substrates.
[0008] Dielectric materials are commonly used as insulating layers
between circuits, and layers of circuits in multilayer integrated
circuits, the most commonly used of which is silica, which in its
various modifications has dielectric constants of the order of
3.0-5.0, more usually 4.0-4.5. Low values of dielectric constant
are preferred and organic polymers inherently usually have low
dielectric values in the range 1.9-3.5, so that considerable
research and work has been done to try to develop polymers suitable
for this special purpose, among which are polyimides (frequently
fluorinated), PTFE, and fluorinated poly(arylene ethers), some of
the materials having dielectric constants as low as that of air,
i.e. 1.00. At the present time fluorination is the most common
modification of the polymers employed in view of the improvements
obtained comprising lowered dielectric constants, enhanced optical
transparency, and reduced hydrophilicity and solubility in organic
solvents, but the fluorination usually results in the polymers
exhibiting a degree of polarization which can cause problems in
obtaining the desired dielectric properties.
[0009] U.S. Pat. No. 5,658,994, issued Aug. 19, 1997, and U.S. Pat.
No. 5,874,516, issued Feb. 23, 1999, both to Air Products and
Chemicals, Inc. of Allentown, Pa., the disclosures of which are
incorporated herein by this reference, disclose and claim a unique
utility as a dielectric coating material for micro-electronic
devices of a class of poly(arylene ethers) as a replacement for
silica-based dielectric materials, wherein the poly(arylene ether)
does not have nonaromatic carbons (other than perphenylated
carbon), fluorinated substituents or significantly polarizable
functional groups. These materials, which are relatively easily
synthesized, are found surprisingly to have an excellent
combination of desirable properties, namely thermal stability, low
dielectric constant values, low moisture absorption and low
moisture outgassing.
[0010] U.S. Pat. No. 5,658,994 discloses and claims in its broadest
aspect an article of manufacture comprising a combination of a
dielectric material and a microelectronic device, wherein the
dielectric material is provided on the microelectronic device and
contains a poly(arylene ether) polymer consisting essentially of
non-functional repeating units of the structure:
--{--O--Ar.sub.1--O--Ar.sub.2--}.sub.m--{--O--Ar.sub.3--O--Ar.sub.4--}.sub-
.n--
[0011] wherein m=0 to 1.0; and n=1.0-m; and
Ar.sub.1,Ar.sub.2,Ar.sub.3 and Ar.sub.4 are individually divalent
arylene radicals selected from the group consisting of: phenylene;
biphenyl diradical; para-terphenyl diradical; meta-terphenyl
diradical; ortho-terphenyl diradical; naphthalene diradical;
anthracene diradical; phenanthrene diradical; diradicals of
9,9-diphenylfluorene of specific type; and 4,4'-diradical of
dibenzofuran and mixtures thereof, but Ar.sub.1, Ar.sub.2,
Ar.sub.3, and Ar.sub.4, other than the diradical
9,9-diphenylfluorene, are not isomeric equivalents.
[0012] U.S. Pat. No. 5,874,516 claims poly(arylene ether)
consisting essentially of non-functional repeating units of the
structure:
--{--O--Ar.sub.x--O--Ar.sub.1--}.sub.m--{--O--Ar.sub.2--O--Ar.sub.3--}.sub-
.n--
[0013] wherein m=0.2 to 1.0; and n=1.0-m; and Ar.sub.1,
Ar.sub.2,and Ar.sub.3 are individually divalent radicals selected
from the group defined in the preceding paragraph; or essentially
of non-functional repeating units of the structure:
--{--O--Ar.sub.X--O--Ar.sub.1--}.sub.m--{--O--Ar.sub.X--O--Ar.sub.3--}n--
[0014] wherein m=0 to 1.0; and n=1.0-m; Ar.sub.X is a special
radical 9,9-bis(4-oxyphenyl)fluorene and Ar.sub.1 and Ar.sub.3 are
individually divalent radicals also selected from the group defined
in the immediately preceding paragraph.
[0015] Variations in Ar.sub.1, Ar.sub.2, Ar.sub.3 and Ar.sub.4 are
stated to allow access to a variety of properties such as reduction
or elimination of crystallinity, modulus, tensile strength, high
glass transition temperature, etc. The polymers are said to be
essentially chemically inert, have low polarity, have no additional
functional or reactive groups, and to be thermally stable at
temperatures of 400.degree.-450.degree. C. in inert atmospheres. In
addition to the basic polymer structures as outlined above the
polymers may also be cross-linked, either by cross-linking itself,
through exposure to temperatures in the range of
350.degree.-450.degree. C., or by providing a cross-linking agent,
as well as end capping the polymer with known end capping agents,
such as phenylethynyl, benzocyclobutene, ethynyl and nitrile. The
ability to crosslink at elevated temperatures, with the consequent
marked increase in molecular weight and density makes the materials
particularly useful in microelectronic applications because they
can readily be applied from solution and then converted to a
solvent resistant coating by heating.
[0016] The specified polymers are non-functional in that they are
chemically inert and they do not bear any functional groups that
are detrimental to their application in the fabrication of
microelectronic devices. They do not have carbonyl moieties such as
amide, imide and ketone, which promote adsorption of water. They do
not bear halogens such as fluorine, chlorine, bromine and iodine
which can react with metal sources in metal deposition processes.
They are composed essentially of aromatic carbons, except for the
bridging carbon in the 9,9-fluorenylidene group, which has much of
the character of aromatic carbons due to its proximity to aromatic
structures; for the purposes of the invention the carbon is deemed
to be a perphenylated carbon.
[0017] The polymers are proposed for use as coatings, layers,
encapsulants, barrier regions or barrier layers or substrates in
microelectronic devices. These devices may include, but are not
limited to multichip modules, integrated circuits, conductive
layers in integrated circuits, conductors in circuit patterns of an
integrated circuit, circuit boards as well as similar or analogous
electronic structures requiring insulating or dielectric regions or
layers. They are also proposed for use as a substrate (dielectric
material) in circuit boards or printed wiring boards. Such a
circuit board has mounted on its surface patterns for various
electrical conductor circuits, and may include various
reinforcements, such as woven nonconducting fibers, such as glass
cloth. Such circuit boards may be single sided as well as double
sided or multilayer.
[0018] It is proposed that additives can be used to impart
particular target properties, as is conventionally known in the
polymer art, including stabilizers, flame retardants, pigments,
plasticizers, surfactants, and the like. It is also proposed that
adhesion promoters can be used to adhere the polymers to the
appropriate substrates. Such promoters are typified by
hexamethyldisilazane, which can be used to interact with available
hydroxyl functionality that may be present on a surface, such as a
silica surface.
DISCLOSURE OF THE INVENTION
[0019] The principal object of the invention is to provide new
methods for manufacturing composite materials consisting of
particles of finely powdered filler material bonded together in a
matrix of polymer material, such new composite materials, and
articles made from such composite materials.
[0020] It is another object to provide such new methods with which
the resultant composite materials and articles comprises at least
60 percent by volume of the filler material, with the remainder
consisting of the polymer material matrix together with any
necessary additives.
[0021] In accordance with the invention there is provided a method
of manufacturing composite materials comprising particles of finely
powdered filler material uniformly distributed in a matrix of
polymer material, the method comprising the steps of:
[0022] mixing together from 60 to 97 volume percent of particles of
the filler material of minimum pore volume when compacted and the
balance of polymer bonding material consisting of nonfunctionalized
poly(arylene ether) to form a composite mixture; and
[0023] subjecting the composite mixture to a temperature sufficient
to melt the polymer material and to a pressure sufficient to
uniformly disperse the melted polymer material into the interstices
between the particles of filler material.
[0024] Also in accordance with the invention there are provided
composite materials comprising particles of finely powdered filler
material uniformly distributed in a matrix of polymer material, the
materials comprising:
[0025] from 60 to 97 volume percent of particles of the compacted
filler material and the balance of polymer material consisting of
nonfunctionalized poly(arylene ether) together forming a uniform
composite mixture;
[0026] wherein the composite mixture has been subjected to a
temperature sufficient to melt the polymer material and to a
pressure sufficient to uniformly disperse the melted polymer
material into the interstices between the particles of filler
material.
[0027] Preferably the polymer material is of maximum dimension or
maximum equivalent spherical dimension of 50 .mu.m.
DESCRIPTION OF THE DRAWINGS
[0028] Methods and apparatus for the production of the new
composite materials, and new composite materials and articles made
of such new composite materials, produced using such methods and
apparatus, that are particular preferred embodiments of the
invention will now be described, by way of example, with reference
to the accompanying diagrammatic drawings wherein:
[0029] FIG. 1 is the first part of a block flow diagram of the
specific method and apparatus for the manufacture of the composite
materials and articles of the invention, particularly for the
manufacture of flat rectangular copper clad substrates intended for
use for electronic circuits;
[0030] FIG. 2 is side elevation of a mixer/solvent evaporation mill
shown in outline in FIG. 1;
[0031] FIG. 3 is a cross-section through the mill of FIG. 2, taken
on the line A-A therein;
[0032] FIG. 4 is another part of the block flow diagram, continuing
from FIG. 1;
[0033] FIG. 5 is a further part of the block flow diagram,
continuing from FIG. 4; and
[0034] FIGS. 6 and 7 are respective part cross sections to a
greatly enlarged scale through a small piece of a typical material
of the invention in order to show the grain structure thereof, and
showing respectively a layer of metal in position to be applied to
a surface, and applied to the surface.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] I have discovered that unexpectedly a particular sub-family
of a known family of polymers, namely poly(arylene ethers), exhibit
unusually high inherent adhesiveness toward finely ground filler
materials of the kind that can be employed in combination with
matrix materials to produce electronic substrates and that, also
unexpectedly, the production of useful composite materials requires
a complete reversal of approach from that which has previously been
employed in the production of composite materials. A major problem
in the prior art processes of forming composite materials, and in
the substrates obtained thereby, is the progressive loss of
mechanical strength that results as the filler solids content is
increased, and hitherto attempts to incorporate more than about 40
volume percent generally has resulted in composite which are so
friable that they literally collapse to a heap of sand-like
material if in testing they are stressed to the degree required in
commercial practice. Moreover, it has been found difficult with
prior art processes to incorporate as much as 40 volume percent
solids material, since the mixtures become so viscous that uniform
mixing is virtually impossible. Consequently, the approach has of
necessity been to incorporate only as much filler material as will
result in a substrate of adequate mechanical strength, and to
accept the lower desired electrical characteristics that result. I
have discovered however that with the methods of the invention, for
the successful production of composite materials, the solids
content must instead be increased to values well beyond those of
the conventional prior art. An acceptable minimum for my new
composite materials is 60 volume percent, in that such materials
are of the required minimum mechanical strength, it being found
that the mechanical strength increases with increased solids
content, instead of decreasing, up to the value of about 95-97
volume percent, or beyond which the proportion of polymer is
reduced below the minimum value required to maintain adequate
adhesion between the uniformly distributed filler particles. It is
my belief that a possible explanation for this highly unexpected
result, although other explanations may be possible, and therefore
I do not intend that the invention be limited thereby, is that
although the chosen polymers exhibit unusually high adhesion,
especially toward oxide materials such as silica, aluminum oxide,
metal powders and boron nitride, they are not particularly
mechanically strong, and therefore are most effective in this new
and special application if employed in the form of very thin
adherent layers interposed between the filler particles, such can
only be obtained with the methods of the invention and when the
solids content is sufficiently high. It is difficult to specify
with any degree of accuracy the optimum thicknesses for the
interposed layers; it is known that layers of 1-3 micrometers are
very successful in giving superior adhesion with adequate strength,
and a possible upper limit is 40 micrometers (0.001 in).
[0036] Composite materials of the invention can be made by mixing
together the required portion by weight, or by volume, of particles
of the chosen non-polar, nonfunctionalized polymer material of
sufficiently small dimension, or equivalent spherical dimension,
e.g. in the range 0.1 to 50 micrometers, with the corresponding
portion by weight or by volume of the chosen filler material, again
of sufficiently small dimension, or equivalent spherical dimension,
e.g. in the range 0.1 to 50 micrometers, and subjecting the mixture
to a temperature sufficient to melt the polymer material, e.g. in
the range 280-400.degree. C. and to a pressure, e.g. in the range
3.5 to 1,380 MPa (500 to 200,000 psi), preferably 70 to 1,380 MPa
(10,000 to 200,00 psi), sufficient to disperse the melted polymer
material into the interstices between the particles of filler
material. By equivalent spherical diameter is meant the diameter of
a completely spherical particle having the same volume as the
specified particle. In alternative processes which are described in
more detail below the polymer may be added in the form of a
solution thereof, provided steps are taken to remove all of the
solvent once the filler and polymer materials have been uniformly
mixed together. The polymer material preferably is selected from
the group comprising polyarylene ether-2, polyarylene ether-3, and
polyarylene ether-4, which materials are described in more detail
below, while the filler material is selected from the group
comprising particles of inorganic material, particles of
electromagnetic material, particles of a core of inorganic material
covered with a layer of a metal oxide material, particles of metal
material particles of magnetic material, and particles of low
dielectric constant high melting point polymer material, all of
which particles may be hollow.
[0037] The resultant heated and pressurized composite mixture may
be formed into a sheet, film or tape, onto a surface of which a
layer of copper may be applied, either by sputtering or by direct
bonding of copper foil under heat and pressure in a vacuum, the
sheet, film or tape being formed by a thermoplastic extrusion
process. Alternatively, green bodies can be cut from sheet or tape
before the heat and pressure step and these green bodies then
converted to heated and pressed bodies by a thermoplastic
compression process, again to a surface of which a layer of copper
can be applied by sputtering or by direct bonding of copper foil
under heat and pressure in a vacuum. The resultant bodies may
comprise substrates for electronic circuits or enclosures for
electronic circuits or devices. The processes of the invention will
be described in detail below in connection with the manufacture of
such thin flat plates, but it will be apparent that they are
applicable also to any shape of molded article with which direct
production of superior surface finish, highly uniform
micro-structures, and high dimensional uniformity from finished
article to article is desired.
[0038] With microelectronic devices, and with the higher
frequencies now employed, the problems of adequate uniformity of
physical and chemical constitution and physical and electrical
properties of the substrates have been exacerbated, and the simple
mixing processes described above usually will not provide
sufficient uniformity, and in such case it becomes necessary to
employ a method and apparatus as described in detail below.
[0039] Referring now to FIG. 1, in this particular process it is
assumed that a mixture of different filler materials are to be
used, especially in view of the opportunity this provides of
specifically tailoring the mechanical and electrical
characteristics of the resultant substrates for the end product.
The polymer is used in the form of a solution thereof (usually of
about 10% concentration) in a suitable solvent such as
cyclohexanone, and the opportunity is taken of employing this
solvent also as a liquid dispersion and suspension vehicle for the
filler materials. A preliminary mixture is first formed of each of
the selected finely powdered filler materials, usually inorganic
materials, with the selected polymer solution, although in other
processes other vehicles may of course also be used. The filler
material or mixture of materials may be obtained respectively by
precipitation or coprecipitation from solutions of suitable
precursors, and however obtained should have the required purity,
dielectric constant, loss tangent, and particle size distribution.
In this embodiment up to four different powdered materials can be
fed from a delivery and metering system comprising a plurality of
hoppers 10, 12, 14 and 16 respectively, while the solution of the
polymer in the cyclohexanone is fed from its hopper 18, and
suitable surface active functional additives, if required, such as
surfactants, plasticizers and lubricants, are fed from a respective
hopper 20. Each powdered material can be fed directly to the
respective hopper 10, 12, 14, and 16, or alternatively obtained
from respective precipitation or coprecipitation systems 22, 24, or
26 (a system for the contents of the hopper 16 is not shown). If
the polymer is employed in the form of a powder then it will be fed
from the hopper 18, while the dispersion vehicle will be fed from a
respective hopper, or perhaps from the hopper 20 along with the
additives. The flow of each filler powder from its hopper is
continuously precision metered by a respective meter 28, that of
the polymer solution is metered by meter 32, that of the surface
active additives is metered by meter 34, and those of the combined
polymer solution/filler or additive flows are metered by respective
meters 36. Each preliminary mixture of polymer solution, powders
and additives is delivered into a respective drum type
mixer/grinding mill 38, described in more detail below.
[0040] One of the aspects of the invention that also distinguishes
it from prior art processes is that it is preferred to use low cost
powders of a relatively wide range of particle sizes in order to
obtain optimum packing together of the particles, and resultant
minimization of the interposed polymer layers, as contrasted with
the highly uniform size, and consequently expensive, powders which
were required, particularly for the production of fired ceramic
substrates to achieve adequate uniformity of processing. Prior to
the formation of each mixture the respective powder particles
usually consist of particles of a range of sizes and agglomerates
of many finer particles that can vary even more widely in size, and
this must be corrected, particularly the reduction of the
agglomerates back to their individual particles. Each mixer/mill 38
operates to mix the components and produce complete dispersion of
the powdered material in the liquid vehicle, and also as a grinding
mill to mill the respective powder particles and agglomerates to a
required size distribution to a obtain a required degree of
uniformity, but with a distribution that will also result in a
minimum pore volume when compacted.
[0041] The proportions of the powder, polymer solution and
additives from the hoppers are such as to obtain a solids content
in the respective preliminary mixture in the range of 40-95% by
volume, the quantities of the dispersing vehicle and the functional
additives being kept as low as possible, but sufficient for the
consistency of the mixture to be kept to that of a relatively wet
paste or slurry, to permit its free flow through the relatively
narrow processing flow passages of the respective mill 38, and the
subsequent machines. A viscosity in the range of about 100-10,000
centipoises will usually be satisfactory. In the methods of the
invention preferably such grinding, deagglomeration and dispersion
of each preliminary mixture is carried out simultaneously in its
respective mill 38, using for this purpose a special mill which is
the subject of my U.S. Pat. No. 5,279,463, issued Jan., 18, 1994,
and U.S. Pat. No. 5,538,191, issued Jul., 23, 1996, the disclosures
of which are incorporated herein by this reference.
[0042] These special mills may be of two major types, in a first of
which the mill has two circular coaxial plate members with a
processing gap formed between them; the axis of rotation can be
vertical or horizontal. It is preferred however to use the second
type of mill, which consists of an inner cylindrical member
rotatable about a horizontal axis inside a stationary hollow outer
cylindrical member, the axes of the two cylinders being slightly
displaced so that the facing walls are more closely spaced together
at one longitudinal location around their periphery to form,
parallel to the axes, what is referred to as a processing or micro
gap, and are more widely spaced at the diametrically opposed
longitudinal location to form, again parallel to the axes, what is
referred to as a complementary or macro gap. The mixture flows
through the processing gap producing so-called "supra-Kolmorgoroff"
mixing eddies in the portion of the slurry at and close to the
macro gap and so-called "sub-Kolmorgoroff" mixing eddies in the
micro or processing gap. Ultrasonic transducers may be mounted on
the stationary member which apply longitudinal pressure
oscillations into the processing gap and reinforce the
"sub-Kolmorgoroff" mixing eddies. Such apparatus is capable of
processing relatively thick slurries of sub-micrometer particles in
minutes that otherwise can take several days in conventional high
shear mixers and ball or sand mills.
[0043] The separate preliminary mixtures are now mixed together to
form a combined mixture having the consistency of a uniform slurry
or wet paste by passing them into a mixer/mill 40, in which the
combined mixture is subjected to another grinding, deagglomerating
and uniform dispersing operation. The mixer/mill 40 is again one of
the above-mentioned special mills which are the subject of my U.S.
Pat. Nos. 5,279,463 and 5,538,191, being also of the type
comprising an inner cylindrical member rotatable inside a
stationary hollow outer cylindrical member. Although only a single
mixer/mill 40 is employed in this embodiment, in some processes it
may be preferred to employ a chain of two or more such mills
depending upon the amount and rate of grinding, deagglomeration and
mixing that is required.
[0044] The milled slurry from the mill 40 passes to a mixer/solvent
evaporation mill 42 which again is of the type comprising an inner
cylindrical member 44 rotatable inside a stationary hollow outer
cylindrical member 46, the paste being carried on the outer
cylindrical surface of the member 44 in the form of a thin film 47.
In the mill most of the cyclohexanone solvent is removed while the
paste is vigorously mixed, the paste becoming continuously thicker
as it travels in a helical path from the feed entry point 48 of the
evaporation mill to its discharge outlet 50 as more and more
solvent is withdrawn through solvent discharge outlet 52, from
which it passes to a condenser (not shown) for recovery and reuse.
The evaporation of the solvent from this mill is facilitated by
heat from a row of cartridge heaters 54 in the base of the machine,
their output being such as to obtain a temperature in the tape body
of about 150.degree. C. Near to the discharge outlet of the mill
the paste is of sufficient stiffness that it can be extruded into a
coherent thin tape 56 of the desired dimension in thickness and
width using a conventional paste extruding machine 58. Since this
tape still contains small amounts of solvent and the additives, it
must be subjected to a further heating process in a tunnel dryer
oven, and to this end the tape is deposited on an endless conveyor
60, which passes it through a drying oven 62, during which passage
the solvent and as much as possible of the additives are removed to
leave the strip or tape consisting only of a thoroughly and
uniformly dispersed composite mixture of the particles of the
filler material or materials and the polymer or polymers. A
suitable temperature for such an oven is, for example, in the range
150-250.degree. C., the heating being carried out slowly to avoid
as far as possible the formation of bubble holes by the exiting
dispersion medium and additives or additive breakdown products.
[0045] The tape 56 of dried paste is passed through a cutting
station 64, in which it is severed into individual "green"
substrate preforms 66, usually of rectangular shape and of the size
required for the electronic circuit board substrate, if that is the
use for which the materials are intended. The preforms are
deposited manually or automatically into the cavity of a heated
compression mold comprising heated upper and lower platens 68 and
70, the cavity being located in the lower heated platen 70 to
facilitate the loading process. Once the preform is loaded the mold
cavity is closed by the downward moving heated top platen 68 which
protrudes into the cavity to compress the preform to its required
dimensions and density. The temperature to which the preforms are
heated in the mold is sufficient to melt the polymer so that it
will flow freely under the pressure applied to completely fill the
interstices and coat the filler material particles, while the
maximum is that at which the ploymer will begin to degrade
unacceptably. The minimum pressure to be employed is coupled with
the choice of temperature, in that it must be sufficient for the
melted polymer to flow as described, the pressure and time for
which the mold is closed being sufficient for the material of the
preforms to attain maximum compaction and density. During the heat
and pressure cycle the melted polymer will flow relatively freely
and the temperature and pressure are maintained for a period
sufficient to ensure that the polymer can completely fill the
relatively small interstices between the solid particles in the
form of correspondingly very thin layers. Typically the temperature
is in the range 280-400.degree. C., while the pressure is in the
range 70 MPa to 1,380 MPa (10,150 to 200,000 psi), although a more
commercially likely pressure is about 345 MPa (50,000 psi), while
pressures as low as 3.5 MPa (500 psi) may be usable. The surfaces
of the platens that contact the preforms are mirror-finished or
better to assist in obtaining the smooth surfaces that are desired
for electronic substrates intended for microwave frequency
applications.
[0046] Another unexpected advantage of the nonfunctionalised
poly(arylene ethers) employed is that, since they may be
cross-linked by exposure to temperatures in the range of
350.degree.-450.degree. C. in the presence of oxygen, it is
possible to take the finished substrate through a cycle in which
initially the polymer is melted again and thoroughly diffused
throughout the body, the polymer at this stage being relatively
fluid, and thereafter the temperature is increased until
cross-linking and corresponding densification of the polymer takes
place. Alternatively, the composite mixture may include as an
additive a cross-linking agent and/or an end capping agent, so that
the desired densification will take place at lower temperatures. As
described above this ability to crosslink and/or end cap at
elevated temperatures makes the materials particularly useful in
microelectronic applications because they can readily be applied as
low viscosity materials, e.g. even from solution as described, and
then converted to a solvent resistant material of maximum density
by the heating.
[0047] The substrates 66 issuing from the press are fed to a
multi-stand, heated, flattening roller mill 72 in which they are
rolled to an accurately controlled thickness and flattened. The
surfaces of these rolls are also mirror-finished, or better, again
in order to obtain the desired final smooth surfaces. The sheet,
film or tape from which the preforms have been cut usually has a
thickness less than about 60 mil, can be less than about 30 mil,
may be less than about 10 mil, may be less than about 4 mil, and
can even be less than about 1 mil. Substrates intended for use in
electronic circuits will usually be of thickness in the range 0.125
mm to 1.5 mm (5-60 mil), and if intended for thick film usage are
usually required to be smooth to about 0.75-0.90 micrometer (22-40
microins), while if intended for thin film usage must be flat to
better than 0.05 micrometer (2 microins). The preforms are now fed
to a heated laminating press 74 in which they are each laminated on
one or both sides with a thin flat smooth piece 76 of copper sheet
of the same size, which subsequently is etched to produce the
electric circuit. These sheet copper pieces are obtained by cutting
from a strip 78 supplied from a roll thereof (not shown) which is
cut into pieces at a cutting and mirror-finish surfacing station
80. The surfacing means comprises a hot press in which the cut
pieces are pressed between a pair of heated platens, the platen
surfaces being mirror-finished or better so that a corresponding
finish is imparted to the surfaces of the pieces. The
mirror-finishing of the substrate surfaces and those of the copper
pieces is especially important in ultrahigh-frequency applications
since, as described above, the currents tend to flow only in the
surface layers of the conductors, and uniformity in characteristics
of the etched conductors is facilitated by such smooth
surfaces.
[0048] With the processes of the invention the volume percentage of
the filler material can be 60% or more, the minimum value being
that at which the interposed layers of polymer are somewhat too
thick to have the required mechanical strength for the substrate to
have the corresponding amount of mechanical strength. The maximum
value is set by the amount of the particular polymer required to
adequately bind the particular filler material to form a strong
coherent body. Thus, they enable the production of composite
materials in which the solids content is easily and economically in
the range 60%-97% by volume, preferably 70%-97% by volume. The
volume fraction of the polymer in the mixtures is only that needed
to adhere the filler material particles together while filling the
pores left in the inorganic powder after its compression to minimum
pore, preferably pore-free, high density. The relatively small
amounts of polymer present in the composite materials must be
extremely well and evenly dispersed among the fine particles, and
this is readily achievable with the processes employed virtually
independently of the particle size of the filler material.
[0049] The process and apparatus described above are particularly
suited for high volume production of composite materials, but
simpler processes requiring less apparatus are also within the
scope of the invention. For example, as described above it is also
possible to mix together the finely divided filler material and
polymer, the dispersion medium, and its necessary additives and
thereafter rely upon its processing in one or a series of
mixer/mills 38 and/or 40 to produce the required thorough
dispersion, while at the same time obtaining the preferred range of
particle sizes, the dispersed mixture that is produced thereafter
being passed to the drying oven 46 etc., as with the prior
process.
[0050] In many applications the degree of uniformity required in
the material of the finished substrate is such that even the
extensive specific process described above may not be sufficient,
and it may be necessary to apply an additional series of steps in
which the substrates are broken and ground back down to about the
original particle size distribution, with the difference that the
filler material particles are now intimately associated with
particles and thin coatings of the polymer. This finely divided
material is again ground and dispersed in a suitable medium by use
of one or a chain of the special mills, such as the mills 38 and 40
described above, until the maximum possible uniformity is obtained,
when the dispersion medium is removed and the resultant material
again subjected to a heating and pressing operation to produce the
desired substrates, the polymer being sufficiently thermoplastic at
the temperatures required for this to be possible.
[0051] FIGS. 6 and 7 are respective photo micrograph cross sections
through a material of the invention, respectively before and after
the mirror finished piece 76 of copper sheet is attached to the
mirror-finished surface of the substrate, the material consisting
of closely packed particles 82 of the filler material, of irregular
size and shape, coated and bound together by polymer material 84
that no longer exists as discrete particles but as thin intervening
films and interstice-filling masses. As an indication of the size
of the particles, etc. involved the square section of FIG. 6
measures 5 micrometers each side. The adhesiveness of the polymers
of the invention are sufficient to ensure adequate bonding without
the need for reinforcing fibers or fiber-cloth.
[0052] A particular currently preferred group of the selected
poly(arylene ether) polymers, in which the repeating unit is
biphenyl diradical linked with the 4,4'-diradical of
9,9-diphenylfluorene, are designated PAE-2, while another currently
preferred group, in which the repeating unit is para-terphenyl
diradical linked with the 4,4'-diradical of 9,9-diphenylfluorene,
are designated PAE-3, and third currently preferred group, in which
the repeating unit is a combination of the units of PAE-2 and
PAE-3, are designated PAE4. Methods for the production of these
polymers are disclosed in the above-mentioned U.S. Pat. Nos
5,658,994 and 5,874,516, to which reference may be made. Samples of
these polymers are found to have the following principal
characteristics:
1 PAE-2 PAE-3 PAE-4 Weight average molecular 65,300 45,400 75,800
weight Mw Number average molecular 20,700 11,400 25,700 weight Mn
Mw/Mn 2.58 3.98 2.95 Glass transition temperature .sup. 257.degree.
C. .sup. 271.degree. C. .sup. 273.degree. C. Tg via DSC Tensile
modulus (dynes/cm.sup.2) 1.45 .times. 10.sup.10 1.45 .times.
10.sup.10 .39 .times. 10.sup.10 Weight loss % at 400.degree. C.
0.36 0.57 0.65 after 6 hrs Weight loss % at 450.degree. C. 0.91
1.65 1.26 after 6 hrs Wt % gain moisture at 0.279 0.301 0.274
85.degree. F./85RH
[0053] In the above-mentioned U.S. patents these materials are
described as having improved properties, as compared with prior art
fluorinated poly(arylene ether) materials designated PAE-1, a
particular sample of which has the following comparable
characteristics:
2 PAE-1 Weight average molecular weight Mw 20,000 Number average
molecular weight Mn 7,700 Mw/Mn 2.58 Glass transition temperature
Tg via DSC .sup. 166.degree. C. Tensile modulus (dynes/cm.sup.2)
1.23 .times. 10.sup.10 Weight loss % at 400.degree. C. after 6 hrs
0.72 Weight loss % at 450.degree. C. after 6 hrs 3.16
[0054] Substrates made using PAE-2 have been very successful; the
material does not oxidise in air, is highly adhesive without the
use of coupling agents, and has a loss tangent in the frequency
range of particular interest (1-10 GHz) less than 0.0008, as
compared to most other thermoset polymer materials presently used
for electronic circuit applications, namely 0.02-0.005. The polymer
is thermoplastic and can be processed at 280-300.degree. C., and by
post treating the substrates at 300-400.degree. C. to establish
cross-linking they can be renedered thermoset, when the loss
tangent drops below 0.0008. Polymers of weight average molecular
weight below about 30,000 are regarded as less desirable for use
with the methods of the invention, since even more than the
PAE-2/3/4 materials they are not able to form adequately
structurally strong films, sheets or any other substantially
three-dimensional body, because of a tendency of these relatively
thick structures to shatter into a multitude of smaller fragments.
I have discovered however that surprisingly even the lower
molecular weight materials remain intact and cohesive as thin film
depositions in the low micrometer range thicknesses of about 1-3
micrometers and can therefore be used, although the higher
molecular weight materials are to be preferred.
[0055] The relative proportions of the filler materials and of the
polymer depend at least to some extent upon the use to which the
substrate is to be put; if a very high frequency circuit is to be
installed then it will be preferred to have the maximum amount of
filler dielectric material and the minimum amount of polymer. As
has been described above, the minimum amount of polymer is set by
that required to fill the intergrain interstices when the
interstitial volume is at its minimum value, and to ensure
sufficient coating of the grains for the resulting composite to
have the required mechanical strength. For this reason the
composites usually require a minimum of 3% by volume of polymer to
be present as long as the optimum particle packing of the filler
material has been obtained, the remaining 97% solids content
comprising the filler dielectric material, residual surface active
and coupling agents, and organic or inorganic reinforcing,
strength-providing fibers and whiskers, when these are
provided.
[0056] Materials of relatively small particle sizes are preferred,
particularly for the filler starting materials, and also for the
polymer if a solid polymer or polymers is employed. The preferred
particle size range for the filler starting materials is from 0.01
to 50 micrometers, while that for the polymer is from 0.1 to 50
micrometers. As described above, the presence of particles of
filler material of a range comprising different sizes is preferred,
since this improves the capability of dense packing in a manner
that reduces the interstitial volume, and consequently facilitates
the production of the very thin highly adhesive layers that are
characteristic of the invention, besides reducing the amount of
polymer required to fill the interstices and adhere the particles
together. It can be shown theoretically that the minimum
interstitial volume that can be obtained when packing spheres of
uniform size is about 45%. Owing to the wider particle size
distribution that can be employed, this volume can be reduced
considerably further, down to the specified value of about 3%.
[0057] As described above, there are a number of important
parameters for the resultant substrates which must be considered in
making a selection of the fillers and polymers to be used. Among
those which require the highest possible values are tensile
strength; peel strength; solder joint reliability; compliance i.e.
low modulus; plated through hole reliability; dielectric constant;
chemical inertness; dimensional stability and Q factor. Among those
which require the lowest possible values are water absorption,
crosstalk v line spacing, and loss tangent or dissipation factor
(reciprocal of Q factor).
[0058] The methods of this invention are particularly applicable to
the production of composite materials in which the finely powdered
filler material consists of any one or a mixture of the "advanced"
materials that are now used in industry for the production of fired
ceramic substrates for electronic circuits, the most common of
which are aluminium nitride; barium titanate; barium-neodymium
titanate; barium copper tungstate; lead titanate; lead magnesium
niobate; lead zinc niobate; lead iron niobate; lead iron tungstate;
strontium titanate; zirconium tungstate; the chemical and/or
physical equivalents of any of the foregoing; alumina; fused
quartz; boron nitride; metal powders; and semiconductors. Another
important group is compositions comprising powdered ferrites and
like inductive materials in a polymer matrix have already been
produced, used for example in magnetic passive products such as
transformers, inductors and ferrite core devices, but the methods
used add the powdered filler material into the polymer matrix and
their solids contents have generally been limited to not more than
about 50% by volume. The invention permits the production of such
composite materials of higher solids content, e.g. 80% by volume
and above.
[0059] At this time the only ceramic materials with temperature
stable dielectric constants that are available have values in the
ranges 2.6 to 12, 37 to 39 and 80 to 90, whereas in the quickly
expanding market of wireless telecommunication, which is based on
microwave frequencies ranging from 800 MHz to over 30 GHz, and in
which small size and low weight are of increasing importance, the
preferred dielectric constant values need to be tailored to be
anywhere between 8 and 2000, according to choices dictated by the
optimum circuit architecture, instead of, as at present, the
circuit architecture being dictated by the very limited ranges of
dielectric constants that are available. In microwave or GHz
frequencies signal propagation depends mainly on the waveguide
character of the circuitry and consequently only such high
dielectric constant materials allow significant miniaturization,
permitting the use of narrower conductor line widths and shorter
lengths. For example, coaxial dielectric resonators, at this time
used in more than 25 million cellular telephones worldwide, could
be reduced in size and weight by more than half and in cost by more
than two thirds if the dielectric constant of the substrate
material could be raised from the present value of alumina of about
9 to over 400 and its dielectric loss (loss tangent) kept below
0.0005.
[0060] It is possible with these processes to fabricate composite
materials in which the powdered filler material is a tailored blend
of two or more individual materials. The requirements for substrate
materials, especially for very high frequency applications, are
very exacting, requiring consideration of a large number of
physical properties including filler material content, bulk density
(range), surface finish, grain size (average), water absorption(%),
flexural strength, modulus of elasticity, coefficient of linear
thermal expansion, thermal conductivity, dielectric strength,
dielectric constant, dissipation factor, and volume resistivity.
The possibility of such blending makes it possible to tailor the
properties of the substrates to their specific tasks in a manner
which is not possible with a sintered ceramic as in most cases the
sintering phase rules would be violated and the resulting fired
material would fall apart. One of the main reasons for combining
filler materials in any given ratio is to obtain a mixture with a
tailored dielectric constant, which constant will remain uniform
over a temperature range from say -50.degree. C. to +200.degree.
C., and with a very high Q factor (equivalent to a very low loss
tangent) desirably above 500 and if possible as high as 5,000.
* * * * *